Chemistry Reactions
Key Concepts & Formulas
| # | Concept | Quick Explanation |
|---|---|---|
| 1 | Redox Reaction | Oxidation = loss of e⁻ (OIL); Reduction = gain of e⁻ (RIG) |
| 2 | Exothermic vs Endothermic | Exo: heat given out (ΔH –ve); Endo: heat absorbed (ΔH +ve) |
| 3 | Catalyst | Substance that speeds reaction but itself is not consumed; lowers activation energy |
| 4 | pH scale | 0–6.9 acidic, 7 neutral, 7.1–14 basic; pH = –log[H⁺] |
| 5 | Displacement rule | More reactive metal displaces less reactive from its salt solution (Zn + CuSO₄ → ZnSO₄ + Cu) |
| 6 | Law of Conservation of Mass | Total mass of reactants = total mass of products; balance atoms on both sides |
| 7 | Common gases test | CO₂ → lime-water milky; O₂ → rekindles glowing splint; H₂ → ‘pop’ sound with lighted match |
10 Practice MCQs
1. Which of the following is an endothermic process?
A. Burning of coal B. Respiration C. Melting of ice D. NeutralisationAnswer: C
Solution: Melting requires heat absorption, hence ΔH is positive.
Shortcut tip: “Phase change solid→liquid/gas usually endo.”
Concept tag: Thermo-chemistry, Endothermic
2. In the reaction 2Mg + O₂ → 2MgO, the species oxidised is:
A. O₂ B. Mg C. MgO D. NoneAnswer: B
Solution: Mg loses electrons (0 → +2); oxidation.
Shortcut tip: “OIL—Oxidation Is Loss (of e⁻).”
Concept tag: Redox
3. The colour of ferrous sulphate solution changes from green to brown on standing due to:
A. Reduction B. Oxidation of Fe²⁺ → Fe³⁺ C. Hydration D. DehydrationAnswer: B
Solution: Fe²⁺ oxidises to Fe³⁺ (brown).
Shortcut tip: “Green Fe²⁺ → Brown Fe³⁺ by air.”
Concept tag: Oxidation states
4. A catalyst increases the rate of reaction by:
A. Increasing activation energy B. Decreasing activation energy C. Increasing enthalpy change D. Increasing equilibrium constantAnswer: B
Solution: It provides an alternate path with lower Eₐ.
Shortcut tip: “Catalyst = Shortcut, not destination.”
Concept tag: Catalysis
5. pH of lemon juice is about 2, therefore its H⁺ ion concentration is:
A. 10⁻² M B. 10² M C. 2 M D. 12 MAnswer: A
Solution: pH = –log[H⁺] ⇒ [H⁺] = 10⁻² M.
Shortcut tip: “pH number = negative exponent of [H⁺].”
Concept tag: pH scale
6. When Zn strips are added to blue CuSO₄ solution, the colour fades because:
A. Cu is precipitated B. ZnSO₄ is white & Cu is displaced C. Solution becomes acidic D. Zn is coated with CuAnswer: B
Solution: Zn (higher reactivity) displaces Cu; CuSO₄ (blue) → ZnSO₄ (colourless).
Shortcut tip: “Higher metal kicks out lower metal.”
Concept tag: Displacement
7. Which gas turns lime-water milky?
A. O₂ B. CO C. CO₂ D. NH₃Answer: C
Solution: CO₂ + Ca(OH)₂ → CaCO₃↓ + H₂O (milky).
Shortcut tip: “CO₂ = chalk maker.”
Concept tag: Gas test
8. The balanced form of ___C₂H₆ + ___O₂ → ___CO₂ + ___H₂O uses coefficient of O₂ as:
A. 3.5 B. 5 C. 7 D. 7⁄2Answer: D
Solution: C₂H₆ + 7⁄2 O₂ → 2CO₂ + 3H₂O. (Multiply throughout by 2 for integers.)
Shortcut tip: “Balance C→H→O; keep O₂ last.”
Concept tag: Balancing
9. Which reaction is both precipitation & double displacement?
A. AgNO₃ + NaCl → AgCl↓ + NaNO₃ B. NH₄NO₃ → N₂O + 2H₂O C. C + O₂ → CO₂ D. 2H₂O₂ → 2H₂O + O₂Answer: A
Solution: Exchange of ions & formation of precipitate.
Shortcut tip: “↓ means both double disp + ppt.”
Concept tag: Types of reactions
10. Respiration is considered exothermic because:
A. CO₂ is released B. O₂ is consumed C. Energy & heat are produced D. Water is formedAnswer: C
Solution: Glucose breaks, releasing heat (ΔH –ve).
Shortcut tip: “Body feels warm → exo.”
Concept tag: Bio-chemistry
5 Previous Year Questions
1. The reaction 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ is: [RRB NTPC 2021]
A. Combination B. Decomposition C. Displacement D. RedoxAnswer: B
Solution: One reactant breaks into many → thermal decomposition.
Shortcut tip: “One → many = Decomp.”
Concept tag: PYQ, Reaction type
2. Which metal does NOT react with cold water? [RRB Group-D 2019]
A. Na B. K C. Ca D. FeAnswer: D
Solution: Iron needs steam; Na/K/Ca react with cold water.
Shortcut tip: “Fe = lazy in cold water.”
Concept tag: PYQ, Metal activity
3. The brown-ring test is used for the detection of: [RRB ALP 2018]
A. Nitrate B. Sulphate C. Chloride D. BromideAnswer: A
Solution: Conc. H₂SO₄ + FeSO₄ → [Fe(NO)]SO₄ brown ring if NO₃⁻ present.
Shortcut tip: “Brown ring = NO₃⁻.”
Concept tag: PYQ, Anion test
4. Electrolysis of brine gives: [RRB NTPC 2016]
A. H₂, O₂ B. H₂, Cl₂, NaOH C. Na, Cl₂ D. Only Cl₂Answer: B
Solution: 2NaCl + 2H₂O → H₂ (cathode) + Cl₂ (anode) + 2NaOH (aq).
Shortcut tip: “Brine = House of H₂, Cl₂, NaOH.”
Concept tag: PYQ, Electrolysis
5. Rusting of iron is an example of: [RRB JE 2015]
A. Reduction B. Oxidation C. Corrosion + Oxidation D. DissociationAnswer: C
Solution: Fe → Fe³⁺ + e⁻ (oxidation) in presence of O₂ & H₂O → corrosion product.
Shortcut tip: “Rusting = Fe losing electrons.”
Concept tag: PYQ, Everyday chemistry
Speed Tricks & Shortcuts
| Situation | Shortcut | Example |
|---|---|---|
| Decide oxidised/reduced species | “OIL RIG” in 2 sec | Oxidation Is Loss; Reduction Is Gain of e⁻ |
| Predict reactivity order | “Please Stop Calling Me A Careless Zebra …” → Pb, Sn, Cu, Hg, Ag, Au (less reactive) | Any metal above H displaces it from acid |
| Gas evolved & test | “COP” → CO₂ = milky; O₂ = glowing splint rekindles; Pop = H₂ | Mnemonic: COP stations |
| Balancing combustion of CₓHᵧO₂ | 1×CₓHᵧ + (x + y/4 – z/2) O₂ → x CO₂ + y/2 H₂O | Saves 30 s on rough work |
| pH to [H⁺] | Remove ‘p’, put 10^ (negative) | pH 4 ⇒ [H⁺] = 10⁻⁴ M |
Common Mistakes to Avoid
| Mistake | Why Students Make It | Correct Approach |
|---|---|---|
| Writing O₂ as O (mono-atomic) | Rush & forget diatomic gases | Remember “HONClBrIF” are diatomic |
| Balancing H before O | Leads to fractions | Always balance C → H → O last |
| Calling every precipitate ‘ppt’ without ‘↓’ | Ignoring state symbol | Always note (s) or ↓ in equation |
| Forgetting catalyst is unchanged | Thinking it’s a reactant | Write it above arrow, exclude from net equation |
Quick Revision Flashcards
| Front | Back |
|---|---|
| What is oxidation in terms of electrons? | Loss of electrons (OIL) |
| Product of complete combustion of hydrocarbon? | CO₂ + H₂O |
| Colour of phenolphthalein in basic medium? | Pink |
| Formula of rust? | Fe₂O₃·xH₂O |
| Catalyst in Haber process? | Finely divided Fe + Mo promoter |
| Acid present in vinegar? | Acetic acid (CH₃COOH) |
| Gas with choking smell & reddish-brown colour? | NO₂ |
| Precipitate colour when BaCl₂ added to SO₄²⁻? | White (BaSO₄) |
| Test for H₂ gas? | Burning splint gives ‘pop’ sound |
| pH range of human blood? | 7.35 – 7.45 (slightly basic) |
BETA
